FR 2 534 222 makes known a counter-torque device with ducted tail rotor and ducted flow-straightening stator for a helicopter, including a duct, of axis substantially transverse to the helicopter, and in which are located, on the one hand, the rotor, substantially coaxial with said duct and generating a transverse flow of air, and, on the other hand, the flow-straightening stator, including a plurality of profiled vanes fastened inside said duct downstream of the rotor with respect to the flow of air, and arranged in a substantially radial fashion with respect to said duct so as to be able to recover rotational energy from the flow of air at the exit from the rotor in the form of a counter-torque axial thrust adding to that produced by the rotation of the rotor within the duct, without increasing the power transmitted to the rotor for this purpose.
French Patent FR 2 534 222 also proposes for the vanes of the flow-straightener to extend between a stationary central body substantially coaxial in the duct and the annular wall of the fairing duct, and for them to provide alone the mechanical link between this stationary body and said wall, and therefore fastening of the stationary body and of the rotor, mounted so that it can rotate on this body, within the duct, as replacement for support arms, often numbering three, generally used to provide this mechanical link between the stationary body and the wall of the duct.
It is further proposed in the abovementioned patent for the duct to exhibit, from the upstream end to the downstream end, a convergent inlet with rounded edge, a cylindrical part, in which the blades of the rotor rotate and a divergent nozzle ending in a divergent outlet, the flow-straightener being located within the divergent nozzle, and it being possible for the vanes which constitute it to be independent of one another and to have a constant aerodynamic profile and zero twist, for example a profile of the NACA 65A10 type. Such a flow-straightener with profiled stationary vanes simultaneously fulfills several functions and gives significant advantages:
from the aerodynamic and acoustic standpoints, the vanes straighten out the airflow leaving the rotor and thus enable some of the rotational energy of the airflow to be converted into additional counter-torque thrust, and the replacement of the known support arms [most often cylindrical and of relatively sizeable diameters, bathed in the flow from the rotor and constituting one of the significant sources of noise of the device by means of the noise of interaction between the rotor and the support arms] with profiled vanes decreases the drag while reducing the acoustic emission of the ducted device;
from the standpoints of the transmission of loading and rigidity, the flow-straightener connecting the structure of the helicopter to the mechanical assembly mounted in the duct makes it possible to transmit all the loadings from the hub of the rotor, from the rear transmission box (for the rotational drive of the rotor) and from the collective pitch control of the blades of the rotor to the wall of the duct, these loadings being better distributed by the vanes in the fairing, so that the use of ribs for taking up loading in the latter proves pointless, the flow-straightener increasing the rigidity of the assembly owing to the fact that its vanes provide more numerous points for taking up loads on the wall of the duct than a conventional tripod of support arms.